The present invention relates generally to electrical power distribution apparatus. More particularly, the invention relates to current limiting fuses and to dropout fuseholders for spanning the gate between the spaced-apart terminals of an overhead distribution cutout mounting. Still more particularly, the invention relates to a new current limiting dropout fuseholder adapted for installation in the industry-standard interchangeable cutout mountings that are presently used with expulsion fuses.
A fuse is a current interrupting device which protects a circuit by fusing open its current-responsive element when an overcurrent or short-circuit current passes through it. A fuse has these general functional characteristics: (1) It combines both a sensing and interrupting element in one self-contained device; (2) It is direct acting in that it responds only to a combination of magnitude and duration of current flowing through it; (3) It normally does not include any provision for making or breaking the connection to an energized circuit but requires separate devices to perform this function; (4) It is a single-phase device, such that only the fuse in the phase or phases subjected to overcurrent will respond to de-energize the affected phase or phases of the circuit that is faulty; (5) After having interrupted an overcurrent, it is replaced to restore service.
Fuses are typically employed in the electrical utility industry to protect distribution transformers, cables, capacitor banks and other equipment from damaging overcurrents. The fuses are arranged to disconnect the faulted equipment or circuit promptly from its source of supply before damage can occur.
At the present time, two basic types of fuses are employed, the expulsion fuse and the current limiting fuse. Each type employs a fusible element designed to melt when a current of a predetermined magnitude and duration passes through the element.
Expulsion type and current-limiting type fuses effect interruption of overcurrents in a radically different manner. The expulsion type fuse interrupts overcurrents through the deionizing action of gases that are liberated when the fusible element melts. The current-limiting type interrupts overcurrents when the arc that is established by the melting of the fusible element is subjected to the mechanical restriction and cooling action of a sand filler that surrounds the fusible element.
The earliest of these two types of fuses was the expulsion fuse. An expulsion fuse typically employs a relatively short length of a fusible element (within what is commonly termed a "fuselink") contained within a tubular enclosure that is part of a larger assembly known as a "fuseholder". The enclosure used in the expulsion type fuse is lined with an organic material, such as bone fiber. Interruption of an overcurrent takes place within the fuse by the deionizing and explosive action of the gases which are liberated when the liner is exposed to the heat of the arc that is created when the fusible element melts in response to the overcurrent. The operation of the expulsion-type fuse is characterized by loud noise and violent emission of gases, flame and burning debris, all of which pose a danger to personnel who may be in close proximity to the fuse when it operates. Because of its violent mode of operation, this type of fuse has generally been restricted to outdoor usage only. Even when employed outdoors, the expulsion fuse must be mounted well away from the equipment it is intended to protect, as well as other equipment, due to the explosive nature of its operation and its tendency to inject ionized gases into insulating spaces. Further, expulsion fuses mounted on distribution system poles have been known to initiate grass fires resulting from the flaming debris which may be expelled.
Another inherent disadvantage of the expulsion type fuse is that it requires 1/2 or sometimes 1 full cycle of current before the fuse clears a high current fault. During this time, the equipment the fuse is designed to protect must endure the full available fault current that is allowed to pass through the fuse to the equipment. Potentially damaging energy that will be dissipated in the equipment will be proportional to the formula I.sup.2 T, where I is the magnitude of the overcurrent and T is the time that the current condition exists. Additionally, the high current that an expulsion fuse allows to flow prior to its interruption at a system current zero tends to cause bothersome voltage dips upon the network causing lights to flicker and sensitive computers and electronic equipment to suffer. Further, expulsion fuses may not clear the overcurrent condition soon enough to prevent sectionalizing fuses, reclosers or other protective relays and circuit devices from also sensing the overcurrent and responding by temporarily and sometimes permanently disconnecting other portions of the network. Additionally, the increased demand for electrical service has led to lower impedance distribution networks and the need for greater interrupting capabilities, sometimes capabilities which exceed those available through the use of expulsion fuses.
The limited interrupting capacity of expulsion-type fuses, coupled with their potentially dangerous mode of operation, their unsuitability for use within buildings or enclosures, their relatively slow clearing time, as well as other factors, prompted the development of the current limiting fuses. The current-limiting fuse has at least three features that have made it extremely desirable for use by the utilities:
(1) Interruption of overcurrents is accomplished quickly without the expulsion of arc products or gases or the development of forces external to the fuse body because all the arc energy of operation is absorbed by the sand filler of the fuse and is subsequently released as heat at relatively low temperatures. This enables the current-limiting fuse to be used indoors, or even in small enclosures. Furthermore, since there is no discharge of hot gases or flame, only normal electrical clearances from other apparatus need to be provided.
(2) A current-limiting action or reduction of current through the fuse to a value less than that otherwise available from the power-distribution network at the fuse location occurs if the overcurrent greatly exceeds the continuous-current rating of the fuse. Such a current reduction reduces the stresses and possible damage to the circuit up to the fault or to the faulted equipment itself, and also reduces the shock to the distribution network.
(3) Very high interrupting ratings are achieved by virtue of its current-limiting action so that current-limiting fuses can be applied on medium or high-voltage distribution circuits of very high available short-circuit currents.
A current-limiting fuse typically consists of one or more silver wire or ribbon elements of a required length which are electrically connected at their ends to a pair of electrical terminations. The assembly--consisting of the fusible element and end terminations--is placed in a tubular housing that is made of a highly temperature-resistant material, and the housing is then typically filled with high-purity silica sand and sealed.
Current limiting fuses require a sufficient minimum element length for proper fault current interrupting performance, and also require sufficient element cross sectional area in order to properly carry the normal or steady-state system currents. To make the fuse shorter than would otherwise be possible, the required element lengths are normally achieved by spirally winding the elements on a support element or "spider" made of a highly temperature-resistant, non-tracking material. Four sided element supports or spiders are most commonly used in current limiting fuses. However, spiders having six or more supporting arms are also used, with some spiders having enough arms such that the element winding approaches a circular shape. A circular shape provides the maximum element length conventionally achieved per turn of winding when the elements are strung directly from supporting arm to the adjacent supporting arm at the appropriate winding pitch. The required cross sectional area of the element is achieved by using ribbon elements up to a maximum practical thickness and, when required for higher current levels, by adding similar elements in parallel. Both paralleling of elements and providing the separation necessary between element turns result in longer spiral windings for the higher current rated fuses.
When operating to clear a high magnitude fault current, the fusible element melts almost instantaneously over its full length. If segments having reduced cross sectional areas are formed in the element, the element melts initially at these reduced area segments, followed by melting of the remaining length of the element. The resulting arc rapidly loses heat energy to the surrounding sand. This energy melts or fuses the sand surrounding the element into a glass-like tunnel structure called a "fulgurite." The rapid loss of heat energy and the confinement of the arc by the molten glass fulgurite literally chokes off the current to a relatively small value. The current is quickly reduced to low levels, brought into phase with the system voltage and interrupted at the earliest occurring current zero of the in-phase current.
Using a metallic ribbon as the high magnitude fusible element is quite common in higher current rated fuses. The ribbon form has the advantage over wire elements of having a larger surface area for thermal conductivity and radiation to the adjacent filler material. Consequently, for a given volume of conductor material, a ribbon element can have a higher steady-state ampere rating than a wire element, as well as improved interrupting characteristics. Ribbon also has the distinct advantage of lending itself to modification with perforations or notches in order to reduce its cross sectional area in order to provide the desired melt characteristics and exact arc-voltage generation control. When a current-limiting fuse using ribbon-type elements encounters a high-fault current, the ribbon portions having reduced cross-sectional area are heated rapidly to the melting point of the ribbon. This produces a fixed number of arclets in series and, thus, limits the magnitude of the arc-voltage spike produced at that instant. The ensuing arc formation continues to vaporize the remaining portions of the ribbon element and finally produces an arc which occupies the full length of the element path.
On low magnitude currents, such as those that might occur from high-impedance faults or sustained overloads, an entirely different phenomenon occurs. In these instances, the element is heated slowly, and ultimately melts in a limited number or perhaps only one place. One or more short arcs begin and attempt to burn back longer sections of the fusible element. The very high heat of the arc again forms a fulgurite. However, because the initial arc length is short and the rate of element burned back may not be fast enough to force an interruption before the highly concentrated heat source destroys the effectiveness of the developed fulgurite, the fuse will fail to interrupt low magnitude currents. Consequently, to achieve interrupting capabilities for low magnitude fault currents, many of today's current limiting fuses employ a second fusible element in series with the primary element, where the second element is designed to fuse open in response to such low magnitude fault currents and to subsequently interrupt these currents.
An important consideration to utilities in fuse selection and use relates to the ability of the fuse to be physically integrated within the utilities existing network and the ease and cost of installation and service. In present-day networks, expulsion fuseholders are typically installed in mountings, the combination of which are known as "cutouts." Generally speaking, a cutout consists of a mounting having an insulating support designed to be mounted on a utility pole or crossarm and having a pair of spaced-apart terminals which are designed to receive and electrically engage a fuseholder or a switch assembly or a combination thereof. When installed, the fuseholder or switch bridges the "gate" between the terminals of the cutout mounting.
The term "fuse cutout" usually refers to the mounting, as described above, when combined with a fuseholder. The fuseholder that is most typically employed in a fuse cutout is designed to be easily disconnected from engagement with the terminals of the cutout. One such fuseholder is the "dropout" type which is designed such that, upon actuation of the fuse, one end of the fuseholder becomes disengaged from the cutout mounting. When this occurs, the unrestrained end of the fuseholder rotates down and away from its normal bridging position between the mounting gate while the fuseholder remains supported from the mounting by its still-engaged end. Fuse cutouts provide protection to the distribution circuit by de-energizing and isolating a faulted section of the circuit.
Expulsion cutouts offer a relatively convenient and low cost means of fusing electrical distribution systems. Further, the industry is adopting a dimensional standard for expulsion fuseholders and mountings, such that a fuseholder from one manufacturer will properly fit into the mounting of another manufacture. Further these "interchangeable" cutouts are widely distributed throughout electrical distribution systems in this country, and large numbers of these cutouts are presently in service.
With increasing demands for electrical energy, more reliable service, higher levels of safety, the need for improved overvoltage protection of transformers and more compact systems, the convenience and cost of fuses with expulsion fuse cutouts do not necessarily meet the needs of today's utilities. Many of the aforementioned problems associated with expulsion fuses can be overcome with current limiting fuses. However, convenience and cost are not adequately addressed with prior art current limiting fusing equipment.
Various attempts have been made to overcome the aforementioned problems as evidenced, for example, by the device disclosed in U.S. Pat. No. 3,827,010 issued to Cameron et al. The device of Cameron provides a combination dropout assembly which includes a current limiting fuse disposed in line, and electrically coupled in series, with an expulsion-type fuse such that a full range of protection is provided by the fuse cutout. However, the overall length of the dropout assembly of this invention is longer than the gate (the spacing between the terminals) of commonly used cutout mountings found in existing distribution systems. Thus, in order to effectively utilize the invention of Cameron et al, utilities would have to replace literally millions of cutouts presently in service. Such an approach would be prohibitive, not only from the standpoint of equipment cost, but also, and perhaps more significantly, in view of the monumental labor costs associated with the replacement of these cutouts. Further this device does not adequately address or remedy fire hazards, spacing requirements, problems that may result from partial element damage to the series connected device or the miscoordination which could occur when refusing one of the two series-connected sections.
A similar device is illustrated in U.S. Pat. No. 4,011,537 granted to Jackson et al. In this patent, the current limiting fuse and expulsion fuse are each provided with insulating skirts to overcome the flashover tendency sometimes exhibited in devices of this type. Notwithstanding this feature however, the in-line combination dropout assembly of Jackson et al presents the same drawbacks discussed above with respect to compatibility with equipment now in service.
Another approach to overcoming the problems discussed above is illustrated in U.S. Pat. No. 3,863,187 issued to Mahieu et al. Mahieu et al employs an expulsion-type fuse in series with a current limiting fuse, but disposes the latter "outgate" such that it does not form a part of the dropout assembly. One advantage of this construction is that the size of the current limiting fuse is not dictated by the spacing between the terminals of the cutout mounting, and moreover, the full extent of this spacing is available for accommodating the desired length of the expulsion-type fuse. Thus, this arrangement permits full range protection without adversely effecting the overall coordination of the distribution system. However, one shortcoming of the Mahieu device is that replacement of the current limiting fuse is difficult, particularly in adverse weather conditions. In this arrangement, the current limiting fuse is commonly positioned on the source side of the cutout in order to provide the desired operating characteristics. Thus, linemen are usually required to work on an energized portion of the line when replacing the current limiting fuse in the device described by Mahieu et al since utilities seldom, if ever, de-energize the distribution circuit for the purposes of permitting routine maintenance work. This problem is compounded by the fact that there is no method of readily determining whether the current limiting fuse has also operated. Consequently, whenever the expulsion fuse portion of a device, such as shown in the Mahieu et al patent, actuates, recommended practice is to replace both the expulsion fuse and the current limiting fuse, the latter being subsequently tested to determine whether it is suitable for continued service. Further, the danger of a fire being ignited and the added spacing requirements for expulsion-type fuses remain because of gases and parts that will be exhausted from the expulsion fuse. In addition, space, in excess of the normal expulsion fuse requirements, must be allocated for placement of the current limiting fuse. In addition, proper electrical coordination of the two fuses must be maintained in order to ensure indication of a fuse operation and removal of voltage stress across the blown fuse by the dropout action of the expulsion fuse.
Still another fuse cutout is disclosed in U.S. Pat. No. 4,184,138 issued to Beard et al. The apparatus disclosed in Beard et al is another example of a combination of current limiting and expulsion fuse, a combination designed to fit within the existing in-service cutouts. The design of Beard et al contemplates offsetting the axes of the current limiting and expulsion fuses so that the combination will physically fit within existing interchangeable cutouts, making its use more convenient than the apparatus of Cameron et al., Jackson et al or Mahieu et al. However, the invention suffers from many of the disadvantages inherent with the use of expulsion fuses, i.e., noise, expulsion of flaming arc products, coordination requirements, and the like. Additionally, the extra mounting hardware and mounting components for installing the apparatus is cumbersome and therefor undesirable. Further, as explained with respect to the patent to Mahieu et al, upon operation of the expulsion fuse, the current limiting fuse must also be tested, or replaced and later tested, thus eclipsing any significant cost savings.
Accordingly, despite advances made in fuse technology, further advances would be welcomed by the industry. Specifically, due to the increased demand for use of current limiting fuses and the cost-driven necessity of employing existing cutout mountings, there exists a need for a full-range current limiting fuseholder sized so as to fit within the gate of present in-service interchangeable cutout mountings. Ideally, such a fuseholder would be of the dropout variety to provide indication of a fuse operation, to relieve voltage stress across a blown fuse and for ease of installation and maintenance. The current limiting fuseholder would be entirely of the nonexpulsion type to avoid potential danger to personnel, to eliminate the threat of starting a fire, and to allow the apparatus to be safely mounted closer to the protected equipment or to other structures. The current limiting fuseholder would operate without the noise and voltage dip which accompanies expulsion fuse operation. The current limiting fuseholder would greatly minimize or eliminate the potential for violent failure of the protected equipment. A full range non expulsion current limiting fuseholder would eliminate the possibility of miscoordination or problems that can develop from using partially damaged fuses. Both of these are concerns when using an expulsion fuse for the low current clearing part of the fuse package. Further, a full range current limiting fuseholder provides a hermetically sealed environment for both the high and low current interrupting section of the fuse and ensures that interrupting performance will not be adversely affected by contaminating environmental conditions.